PX CT specification

[entropically]

For once we've got a project where I get to specify the CTs rather than just making do with whatever is already there.

The CTs will supply a low-impedance transformer diff relay and I want to specify class PX so that I can ask for a high enough knee point to avoid saturation as much as possible.

I've worked out the fault level, X/R, secondary lead resistance, relay burden etc, but then I get stuck. To specify a suitable Vk for the CT I need to know what the CT secondary resistance is. How do I know what is a reasonable maximum secondary resistance to specify?

I've checked out test certificates from some similar older 5A CTs and it works out to be about 8 mOhm per turn - would this be a reasonable value for a 1A CT or would they use thinner wire to fit in more turns?

Also - is it still common practice to install separate cores for separate protection relays (we're also got a directional relay) or is it acceptable to now supply everything from the same core?

Can anyone recommend a good book on this?

 

[umrpwr]

For a good book on applying CTs take a look at the SEL website.

http://www.selinc.com/zocholl/

Stan Zocholl has a book called Analyzing and Applying Current Transformers

 

[Marmite]

Try this:-

http://www.schneider-electric.com/d...iques-switchgear/general-knowledge/ect194.pdf

It tells you how to specify CT's, although it has a European slant.
Regards
Marmite

 

[slavag]

Hi.
For my pinion, today, with new transformer protection you don't need PX class of CT, it's good for high impedance protection.
I assume, that you in IEC land. Check with relay mnf. what is requirements for CT, I think in lot of case 5P30 30VA
is more from enough, more for the, if you order 1A secondary,you don't have any problem.
Actually, relay mnf. write in the manuals what is n'> ( overcurrent factor) or EMF for the differential, distance protection,etc.
For secondary resistance you can use 0.6-1 Ohm for 5A
and abou 5-7Ohm for 1A.
Regards.
Slava

 

[slavag]

Sorry,
I mean, internal resistance of CT.
Regards.
Slava

 

[scottf]

Slavaq-

Very often we see the Vkp specified as a function of the Rct (CT winding resistance), i.e. Vkp = x * Rct. In fact, it's a good way to do it, as it allows the CT manufacture to explore options to keep the overall cost down.

 

[entropically]

Thanks for the info on typical resistances Slavag. I agree that class PX is probably not really required with modern low impedance relays, but I thought that specifying a class PX could lead to a smaller and cheaper CT that still meets my requirements. With 1A CTs it appears that the CT secondary resistance is going to be way larger than the lead or relay resistance so could easily be the factor that drives the CT into saturation under transient conditions.

I think Scottf's idea sounds like a good plan - leaves designing the CT up to the experts.

Already had the trusty Cahier technique thanks but I've just ordered a copy of Zocholl's book.

 

[slavag]

Hi Entropically.
Of course Skottf are right.
I sent to you typical internal resistance ( CT winding resistance) for first calculation of n'>.
Skottf, we've never specified to CT factory what is Rin of CT we need. You know what is a better and cheaper.
Protection designer always have option change cross section of current wiring.
about 1A.
provide some small calculation.
secondary burden for 5A Rin=0.5Ohm, cable 6mm^2, lenght 100m
S= 0.5*25+0.019*200*25/6=28.3VA
for 1A Rin=8Ohm same cable and lengt
S= 8*1+0.019*200*1/6=8.6VA
That means 1A "more better".
I assume 5P20 20VA will enough.
Regards.
Slava